Last week, Gov. Phil Bredesen of TN proposed one, and the Clinton campaign is now indicating that it is open to considering the idea.
Update: Clinton advisers are guiding me away from thinking that they're seriously considering the plan.
Here's what Clinton herself said this morning:
The governor from Tennessee (Phil Bredesen) suggested that there be a convention of superdelegates, and I think that it is an intriguing idea. I have not considered it long enough to have an opinion on it."